you should have oracle account on support.oracle.com to download the 11.2.0.3.

Oracle Database 11g Release 2
Standard Edition, Standard Edition One, and Enterprise Edition
11/10/11: Patch Set 11.2.0.3 for Linux, Solaris, Windows, AIX and HP-UX Itanium is now available on support.oracle.com. Note: it is a full installation (you do not need to download 11.2.0.1 first). See the README for more info (login to My Oracle Support required).

Hi, you can search Grid Infra installation packages this way from My Oracle Support (MOS) site:
https://support.oracle.com/
log in
write this in "Search Knowledge Base": 753736.1 --> Search
and select: "Quick Reference to Patchset Patch Numbers [ID 753736.1]"
From this document search the Oracle version you needed (11.2.0.3) and pick it Patch Number (10404530 for 11.2.0.3)
Then select "Patches & Updates" tab
Add Patch number (10404530) you find into the Patch Name or Number search field --> press Search button

And select :
11.2.0.3.0 PATCH SET FOR ORACLE DATABASE SERVER
for the platform you are using. --> Download --> Download

There is several packages to download. Grid Infra is usually the third package.

Patch sets are nowdays full installations so you can make clear installation with this.
So there is no need to install first 11.2.0.1 and then patch it. Just install with these packages.